How to increase the current rate of optoisolator(transistor)?

Discussion in 'General Electronics Chat' started by moorejohn90, Dec 22, 2011.

  1. moorejohn90

    Thread Starter Member

    Dec 9, 2011
    57
    0
    Hi,
    I want to know the way of driving optoisolator(optotransistor).
    I know a little about it, but I don't clearly understand.

    First, User need to switch driven LED as before.
    Second, Connect emitter and collector to load.

    Then I start confusing about it.
    1- How many current do I need to supply to driven LED( is like normal LED, Forward Voltage 2V, 20mA).
    2- It can increase base current when I crease supply current to driven LED.
    3- Do I need to connect base connector of optoisolator to voltage of load side(like normal transistor that need to supply Voltage and calculate the require base resistor)
    4- How can I know the collector current, is there any way of measuring collector current with multimeter, because I live in Myanmar and I can't get
    specification document when I bought electric components, they sell component with Model No that is the way of selling electric component.
    5- Do I need to connect another transistor(collector to collector, emitter to emitter) when I want to increase collector current of optoisolator like before.
    John:confused:
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    You will need to get a data-sheet for the device if you want to operate it properly, and know its safe ratings.

    If you can access search engines like Google, you may be able to download the data - but you may have to try a large number of different sites. Many sites charge money for such data, but by searching thoroughly it is often possible to find it free of charge.

    Normally opto-isolator transistors are not biased on, so that light from the LED is required before the transistors will conduct. Often their bases are left open. Sometimes a fairly big resistor is joined base to emitter, to guarantee more complete and faster turning off, but this reduces the available "on" current.

    To obtain higher current output, current from the photo-transistor could fed to the BASE of an output transistor.
     
  3. moorejohn90

    Thread Starter Member

    Dec 9, 2011
    57
    0
    Hi Adjuster,
    "To obtain higher current output, current from the photo-transistor could fed to the BASE of an output transistor. "

    That mean, I need to connect photo-transistor's collector pin to base pin of another transistor.

    Thanks for your reply.
    Regard John.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Whether the output would be taken from the collector or the emitter of the phototransistor would depend on a number of things:

    1. The polarity of the phototransistor (usually NPN).
    2. The type of output transistor used, NPN or PNP.
    3. The sense of output required - ON or OFF when the opto is ON.
    One arrangement to keep the same sense of output with an NPN transistor would use the phototransistor emitter to feed the output transistor base.
     
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